1. Field of the Invention
The invention relates to curable silicone rubber compositions for high-voltage insulators, which cure to give silicone rubbers having a low combustibility. More precisely, the invention relates to addition-crosslinkable or peroxide-crosslinkable silicone rubber compositions containing aluminum hydroxide as filler, with the uncrosslinked composition having a storage stability which is improved compared to the prior art and having improved processing properties as well, these improvements brought about through the use of special additives. These improved properties are achieved both with surface-treated aluminum hydroxide and with untreated aluminum hydroxide.
2. Background Art
Aluminum hydroxide, also known as aluminum trihydrate (ATH), is a stoichiometrically defined, crystalline aluminum hydroxide which is obtainable from precipitates produced by addition of bases to aluminum salt solutions or from bauxite, or an aluminum oxide hydrate, frequently referred to in industry as alumina hydrate, which has the composition Al2O3.H2O or Al2O3.3 H2O and can thus be regarded as a hydrated oxide and can be present as an amorphous constituent of aluminum minerals such as bauxite or alumogel.
Silicone rubber compositions containing aluminum hydroxide powder are already known. It is also known that such compositions cure by means of a curing agent, if appropriate with heating, to form a silicone rubber. The curing agent can be, for example, a peroxide or a combination of a transition metal-containing hydrosilylation catalyst and an organosiloxane comprising methylhydrogensiloxy groups. It is art known that some properties of crosslinked rubber which are necessary for use as, for example, high-voltage insulation or cable sheathing having a low combustibility, for arcing resistance, leakage current resistance and low combustibility, etc., can be improved considerably by addition of a sufficient amount of aluminum hydroxide powder to silicone rubber compositions.
It is known that the use of aluminum hydroxide as a filler brings with it a series of considerable disadvantages. Thus, for example, the processing properties of the uncrosslinked composition, e.g. milled sheet strength and tendency to stick to rollers, markedly deteriorate. As a result of aluminum hydroxide powder having hardly any reinforcing properties, the mechanical properties of the crosslinked rubber, e.g. ultimate tensile strength and elongation at break, become poorer.
The storage stability of the uncrosslinked compositions containing aluminum hydroxide also decreases. “Storage stability” refers to the time over which a composition can be stored under particular conditions, for example at a particular temperature, without the composition prematurely becoming undesirably partially or fully crosslinked or, conversely, without the composition any longer being crosslinkable, for example because components necessary for crosslinking have decomposed.
Both silicone rubber compositions containing untreated aluminum hydroxide and silicone rubber compositions containing aluminum hydroxide whose surface has been treated, for example with a silane, silazane or siloxane, with the aim of improving various properties of the composition or of the rubber, for example processability and storage qualities of the uncrosslinked composition, or dielectric and mechanical properties of the crosslinked rubber, are known to those skilled in the art.
The use of surface-treated aluminum hydroxide was mentioned as early as in the 1970s. U.S. Pat. No. 4,217,466 describes insulators whose screens comprise a silicone elastomer containing surface-treated aluminum hydroxide as filler. The treatment reagent is, for example, a vinylsilane.
In U.S. Pat. No. 5,691,407, addition-crosslinking silicone rubber compositions containing surface-treated aluminum hydroxide are disclosed. The reagent for surface treatment can be a silane or silazane, a titanium compound or a polysiloxane. The use of surface-treated aluminum hydroxide leads to improved electrical properties of the silicone rubber when used in high-voltage insulators. The use of previously surface-treated aluminum hydroxide is preferred, but it is also possible to use untreated aluminum hydroxide in combination with hexamethyldisilazane.
In European patent EP 0 787 772 B1 curable silicone rubber compositions which contain aluminum hydroxide but no further reinforcing fillers and which nevertheless have a good mechanical strength and excellent electrical properties are disclosed. The curing agent is a peroxide. The good mechanical properties are achieved essentially by the aluminum hydroxide powder being treated with a silane or siloxane having alkenyl groups and alkoxy or hydroxy groups, for example vinyltrimethoxysilane or vinyl-containing organosiloxanes having SiOH or Si—OR end groups. The aluminum hydroxide powder may have been pretreated with the reagent, or the treatment can be carried out in situ in the preparation of the silicone rubber composition. Comparative examples in which untreated aluminum hydroxide is used as filler display significantly poorer ultimate tensile strengths and tear propagation resistances. A rubber containing 150 parts of aluminum hydroxide powder per 100 parts of polydiorganosiloxane has an ultimate tensile strength of 5.1 MPa and a tear propagation resistance of 13 N/mm when the aluminum hydroxide has been treated and an ultimate tensile strength of only 1.7 MPa and a tear propagation of 8 N/mm when untreated aluminum hydroxide is used.
European patent EP 0 808 868 B2 discloses curable silicone rubber compositions containing aluminum hydroxide powder which has been surface-treated with an organosilane or organosilazane. The silane or silazane can also contain alkenyl groups, for example vinyltrimethoxysilane or tetramethyldivinyldisilazane. The curing agent can be, for example, a peroxide or a combination of a hydrosilylation catalyst and a polyorganosiloxane containing Si—H groups. The patent states that silicone rubber compositions containing conventional ATH are not stable because the aluminum hydroxide absorbs water and the electrical properties deteriorate as a result. Treatment of the aluminum hydroxide powder with an organosilane or organosilazane is described as central in order to achieve good water resistance and good electrical properties.
European patent EP 0 808 875 B1 describes silicone rubber compositions which can contain aluminum hydroxide and platinum compounds to achieve good flame resistance, these compositions still having sufficient flowability prior to curing to enable them to be processed readily. These compositions comprise polyorganosiloxane having a viscosity in the range from 100 and 100,000 mPa·s, pyrogenic silica, surface-treated zinc carbonate, a polyorganosiloxane containing Si—H groups and a platinum catalyst. Aluminum hydroxide and a further platinum compound are optional constituents of the composition. If aluminum hydroxide is used, it is surface-treated. The surface treatment of the zinc carbonate and of the aluminum hydroxide is critical for achieving the aim of the invention. Since these compositions slowly crosslink even at room temperature, an inhibitor such as ethynylcyclohexanol or triphenylphosphane can be added to prolong the processing time.
U.S. Pat. No. 5,668,205 discloses addition-crosslinking silicone rubber compositions which contain aluminum hydroxide and in addition a trimethylsilyl-terminated dimethylpolysiloxane to improve the electrical properties. Likewise described are peroxidic silicone rubber compositions containing aluminum hydroxide, in which a large part of the organopolysiloxanes bear trivinyl or divinylsilyl end groups and which may optionally further comprise a polysiloxane without unsaturated groups. Such compositions can be processed by injection molding and cured to give silicone rubbers which have improved electrical properties, especially when an insulator produced from the composition is used in an environment with severe atmospheric pollution. The aluminum hydroxide can be untreated or surface-treated. Polymers of this type having divinyl or trivinyl end groups are not customary starting materials in silicone chemistry. Their preparation is expensive and complicated and their use is thus not desirable.
In U.S. Pat. Nos. 5,977,216 and 5,668,205, addition-crosslinking compositions stabilized with ethynylcyclohexanol are disclosed. The keeping qualities of these compositions are not indicated. However, one skilled in the art knows that storage for a period of months will lead to slow crosslinking of the composition if the amount of ethynylcyclohexanol used is not so large as to cause a deterioration in the crosslinking characteristics on processing.
U.S. Pat. No. 5,977,216 states that aluminum hydroxide itself has no reinforcing properties. Thus, if very large amounts of aluminum hydroxide filler are used in the silicone rubber compositions in order to achieve the desired electrical properties, this results in silicone rubbers having low mechanical strength. This patent describes curable silicone rubber compositions containing aluminum hydroxide which has been treated with vinylsilazanes, for example tetramethyldivinyldisilazane, or vinylalkoxysilanes, for example vinyltrimethoxysilane, in such a way that from 1×10−6 to 2×10−4 mol of vinyl groups per gram of aluminum hydroxide are present on the surface. The aluminum hydroxide which has been modified in this way with vinyl groups then has reinforcing properties, so that the strength of the rubber does not suffer despite the high degree of fill. The ultimate tensile strength of the rubbers containing surface-treated aluminum hydroxide is from 45 to 58 kgf/cm2 (corresponding to 4.41-5.69 N/mm2) in the embodiments according to the invention, and from 18 to 25 kgf/cm2 (1.76-2.45 N/mm2) in the embodiments using untreated aluminum hydroxide. The examples describe ready-to-use addition-crosslinking compositions which comprise a hydrosilylation catalyst and an organopolysiloxane containing Si—H groups. To prevent immediate crosslinking of the composition, ethynylcyclohexanol is additionally present as inhibitor. It will be clear to one skilled in the art, that although addition of ethynylcyclohexanol can increase the pot life of the ready-to-use composition to a few days, storage over a number of months is not possible since this would require such a large amount of inhibitor that the crosslinking characteristics during processing would deteriorate.
European published specification EP 0 928 008 A2 describes silicone rubber compositions for high-voltage insulators in which aluminum hydroxide is surface-treated in situ. In the preparation of the compositions, untreated aluminum hydroxide is used in combination with an organosilane coupling agent. The surface of the aluminum hydroxide is hydrophobicized thereby, as a result of which the interaction of the aluminum hydroxide with the polysiloxane is improved and the dispersibility and reinforcing action of the aluminum hydroxide is therefore also improved.
U.S. Pat. No. 6,106,954 discloses addition-crosslinking organopolysiloxane compositions comprising surface-treated aluminum hydroxide, with the treatment reagent being an organosilane or organosilazane or a partial hydrolyzate of these reagents which is free of unsaturated groups. To improve the insulating properties, aluminum hydroxide is used in the silicone rubber. However, since aluminum hydroxide is intrinsically hygroscopic, the silicone rubber looses the insulating properties in a moist environment. As a result of the use of aluminum hydroxide which has been surface-treated as described above, the silicone rubber retains its insulating properties even under moist conditions.
European patent EP 1 037 946 B1 describes addition-crosslinking silicone rubber compositions containing aluminum hydroxide and, as further metal oxide, zinc oxide and optionally titanium dioxide. This composition enables disadvantages of the prior art described in the patent application, e.g. low storage stability and unsatisfactory leakage current resistance, to be overcome. In the example, the aluminum hydroxide is surface-treated in situ by means of an organosilazane. Although the poor storage stability of addition-crosslinking silicone rubber compositions filled with aluminum hydroxide in the prior art is described as one of the disadvantages which is to be overcome by this patent application, the composition is nevertheless prepared as a two component system in which a platinum catalyst is present in one component and a polyorganosiloxane crosslinker containing methylhydrogen units and an inhibitor are present in the other component. The ready-to-use composition produced by mixing the two components is processed immediately.
The addition of organic amines or salts thereof or similar compounds to silicone rubber compositions has been described long ago. In U.S. Pat. No. 3,268,473, ammonium carbonate or ammonium bicarbonate, for example, in combination with organosilicon compounds containing hydroxy groups is added to improve the stiffening behavior (“creep hardening”). U.S. Pat. No. 3,334,062 describes the addition of ammonium hydroxide, ammonium carbonate or ammonium carboxylates to a mixture of finely divided inorganic filler, for example silica, and cyclic organosiloxanes and heating of the mixture to up to 170° C. The process claimed comprises surface treatments of fillers (hydrophobicization) using exclusively cyclotrisiloxanes because of their increased reactivity due to ring stress.
Furthermore, German published specification DE 196 14 343 A1 describes polyorganosiloxane rubber compositions which cure thermally to give elastomers and contain from 0.01 to 1 part by weight of ammonium carbamate, alkali metal carbamate or alkaline earth metal carbamate per 100 parts by weight of a polyorganosiloxane. The carbamate is added to improve the strength of the milled sheet. Fillers present can be both nonreinforcing fillers, i.e. fillers having specific surface areas (BET) of up to 50 m2/g, for example quartz, calcium silicate, metal oxide powders, and reinforcing fillers, i.e. fillers having surface areas (BET) of more than 50 m2/g, e.g. silica or carbon black. The compositions are produced by mixing the constituents and subjecting the mixture to a heat treatment.
The necessity of stabilizing ready-to-use compositions is made clear in, for example, the European patent specification EP 1 077 226 B1. In the case of addition-crosslinking silicone compositions, crosslinking commences in the presence of a catalyst at the moment at which all necessary constituents are present together in one component (ready-to-use composition). The crosslinking rate of the composition then has to be adjusted by means of suitable additives (inhibitors) so that a pot life which is sufficiently long for the intended use results. One-component (ready-to-use) and simultaneously storage-stable compositions offer advantages in terms of logistics, since they have to be produced in finished form only shortly before processing. In the European patent EP 1 077 226 B1, this object is achieved by use of specific platinum catalysts by means of which the pot life is considerably increased compared to the use of customary catalysts. Here too, however, the pot life is limited.
The prior art thus describes a series of advantages in respect of low combustibility and electrical properties when using aluminum hydroxide as filler in silicone rubbers, but also the disadvantages in terms of processing properties and storage stability of the uncrosslinked rubber and mechanical properties of the vulcanized material which aluminum hydroxide brings with it. The prior art attempts to compensate these negative properties essentially by use of surface-treated aluminum hydroxide. However, a considerable disadvantage of such compositions containing surface-treated aluminum hydroxide is that the surface treatment of the aluminum hydroxide powder is an additional complicated and thus expensive process step. The surface treatment of the aluminum hydroxide powder is customarily effected by pretreatment or in-situ treatment with treatment agents such as silanes which are classified as harmful to health or toxic or form toxic substances during processing. Appropriate safety measures are therefore necessary.
It is accordingly advantageous to be able, firstly, to prepare a silicone rubber composition containing untreated aluminum hydroxide which has the same good properties as the compositions containing surface-treated aluminum hydroxide described in the prior art and, secondly, to improve properties such as processability and storage stability when using surface treated aluminum hydroxide.